The embryo implants in the uterus and develops into the baby. It is here that the baby grows until fully developed and ready to come out into the world after nine months. This test, usually done by an ultrasound scan, assesses the uterus to confirm that it will be able to function normally and accept and nourish an embryo till birth.
The endometrium is the lining of the uterus where the embryo implants and grows into a baby. This is what it should look like just before you ovulate (the egg is released).
This picture shows a uterus with a polyp within the endometrium. It is clearly seen as a white lump between the two crosses. As you can imagine, this would prevent the embryo from implanting properly in the uterus.